This invention relates generally to systems for the guidance and control of a spacecraft and, more particularly, of an "upper stage" vehicle employed to carry a payload to its operational orbit. A typical application for an upper stage vehicle is to carry a satellite payload from a low-earth orbit to a geosynchronous orbit. This orbital transition is best accomplished by the use of multiple "burns" of its main rocket engines, each burn providing a desired velocity change to the vehicle. Each burn has to be made when the vehicle is appropriately oriented. Moreover, for a relatively long burn, it is important to control the attitude of the vehicle during the burn, to ensure that the desired velocity change is being imparted to the vehicle.
One approach to attitude control during powered flight is to employ one or more pairs of low-powered engines control the attitude of the vehicle. The low-powered engines perform strictly an attitude control function and the main propulsion engines provide the necessary thrust to change the velocity of the vehicle. The principal difficulty with this approach is that it imposes increased cost and complexity on the vehicle design. An alternative approach to attitude control is to mount the main engines on gimbals, so that the engines may be mechanically pivoted through a limited angular field. With an appropriate pivoting mechanism, the engines can then be used to control the attitude of the vehicle, as well as for imparting velocity changes. Although the need for low-powered engines for attitude control is eliminated by this approach, further complexity is imposed by the need for precise mechanical movement of the main engines.
Another approach that has been proposed is the pulsed modulation of multiple propulsion engines. For example, a pair of engines can be appropriately pulse modulated to control attitude about one transverse axis, such as the pitch axis. Basically, this is a duty-cycle modulation of one or both engines to provide an asymmetrical thrust and a resultant attitude correction torque. The difficulty with this approach is that the engine pulses may interact dynamically with the vehicle structures and set up harmful resonance conditions. This may be especially harmful to sensitive deployed equipment, such as antenna systems.
It will be appreciated from the foregoing that there is still a need for improvement in guidance and control systems for spacecraft. Ideally, what is needed is a system that reduces the cost and complexity of conventional systems, but still provides precise attitude control during powered flight of the vehicle. The present invention satisfies this need.